An orthogonal frequency division multiplexing (OFDM) system and its similar system define resource blocks defined in a time-frequency domain and use the resource blocks as a single unit. FIG. 1 illustrates an example of resource blocks defined in a time-frequency domain, wherein one square block means one resource block, and each resource block is comprised of N number of subcarriers (N is a random number) and M number of subframes (or time durations) (M is a random number).
Meanwhile, in a downlink, a base station allocates at least one resource block to a specific user equipment selected in accordance with a given scheduling rule and transmits data through a corresponding resource block. Also, in an uplink, if the base station selects a specific user equipment in accordance with a given scheduling rule and allocates a resource block to the corresponding user equipment, the corresponding user equipment transmits data to the base station through the allocated resource block.
At this time, if frame loss or damage occurs in the data transmitted to the downlink or the uplink, there are provided an automatic repeat request (ARQ) scheme and a hybrid ARQ (HARQ) method to correct a corresponding error.
According to the ARQ scheme, a receiving side detects a data transmission error occurring on a transmission channel, and if a transmission error occurs in data, requests a transmitting side of retransmission to correct the error. Accordingly, an object of the ARQ scheme is to improve reliability by requesting retransmission of data until the data are transmitted without error if a data transmission error occurs.
The HARQ scheme is obtained by combination of the ARQ scheme and FEC (Forward Error Correction) method. The FEC method corrects an error by using an error correcting code, wherein the error is generated by the channel environment. Reliability in data transmission and data throughput in a mobile communication system can be improved by the HARQ scheme. Particularly, the HARQ scheme is performed between physical layers of the transmitting side and the receiving side, whereby delay in packet transmission can be reduced and transmission efficiency can be improved.
The HARQ scheme is advantageous in that although the receiving side transmits NACK to the transmitting side if it fails to successfully decode a frame, the receiving side stores a frame which is previously received in, a buffer for a certain time period and if the corresponding frame is retransmitted, combines the retransmitted frame with the previously received frame to improve a receiving success rate.
FIG. 2 illustrates the HARQ scheme according to the related art. Referring to FIG. 2, the base station determines a user equipment which will receive a packet and transmission formats (coding rate, modulation mode, data capacity, etc.) used for the packet to be transmitted to the user equipment and transmits control information including the transmission formats to the user equipment through a downlink control channel (S21). At this time, the base station transmits a corresponding data packet to the user equipment (S22). The transmission formats mean combination of at least two of transmission parameters (for example, data bits included in one frame, modulation mode, channel coding rate, channel coding mode, etc.) used in data transmission of the base station or the user equipment. Also, a set of the transmission formats is a set which includes a plurality of different transmission formats, and a transmission format index is an identification number to identify each transmission format within the transmission format set.
The user equipment identifies a transmission format of a packet to be transmitted to itself and a transmission timing point by receiving a downlink control channel and receives a corresponding packet through the transmission format and the transmission timing point. After receiving the packet, the user equipment performs decoding of the packet data. If the user equipment fails to decode the packet data, the user equipment transmits a NACK signal to the base station (S23). The base station which has received the NACK signal senses that packet transmission to the user equipment has been failed and transmits control information for retransmission packet to the user equipment (S24). Then, the base station timely transmits the retransmission packet (S25). The retransmission packet can be transmitted in either the same format as that of the packet transmitted in the step S22 or a new transmission format.
If the user equipment which has received the retransmission packet successfully performs decoding, the user equipment transmits an ACK signal to the base station (S26). The base station which has received the ACK signal senses that packet transmission to the user equipment has been successfully performed and performs the next packet transmission. At this time, the user equipment combines the retransmission packet with the previous packet whose decoding has been failed in various methods such as chase combining and incremental redundancy (IR) combining to perform decoding again.
The aforementioned HARQ scheme can be classified into a channel-adaptive HARQ scheme and a channel-non-adaptive HARQ scheme depending on whether to change an allocation resource in accordance with the channel status during frame retransmission. The channel-non-adaptive HARQ scheme means that retransmission is performed for frame modulation or the number of allocation resource blocks in the same status as that of initial transmission regardless of the channel status. Also, the HARQ scheme can be classified into a chase combining method and an incremental redundancy method depending on whether to transmit a packet having the same format as that of a packet for initial transmission during frame retransmission. The chase combining method which transmits the same packet as that for initial transmission is more advantageous than the incremental redundancy method in that it can be realized more simply.
Meanwhile, if a frequency band occupied by one resource block is sufficiently great in case of frame retransmission, the statuses of respective channels constituting a frequency band within the corresponding resource block may be different from each other. Also, if a coherence time is sufficiently greater than a time interval of retransmission, a channel environment of a frequency domain remains unchanged for a time period of retransmission. For reference, the coherence time represents a time when a channel environment is uniformly maintained for a time period or a time when the channel environment remains unchanged for a certain time period. FIG. 3 illustrates that some subcarriers (first to seventh subcarriers) of a resource block comprised of 25 subcarriers are maintained under the poor channel status in a state that the coherence time is greater than the time interval of retransmission.
In other words, if specific frequency bands (frequency bands of first to seventh subcarriers) within the resource block are under a deep fade status, the corresponding frequency bands are maintained for a time period of retransmission for the above reason. In this case, if the channel-adaptive HARQ scheme and the chase combining method are used for frame retransmission, error continues to occur in the deep fade part. For this reason, a problem occurs in that the corresponding part cannot be corrected through combining in spite of retransmission of several times.
To provide high speed data service of large capacity in a mobile communication system and simplify a structure of a channel, models that can perform packet communication and voice communication through one channel have been developed. In this case, packet data and voice data have different features from each other. In other words, since the voice data are smaller than the packet data in their capacity and are generated relatively periodically at low speed, their occurrence statistical data are close to a circuit. Accordingly, it is necessary to develop a new method considering each feature of the packet data and the voice data when the HARQ scheme is applied to the packet data and the voice data transmitted through one channel.